Most automotive vehicles have an HVAC system that includes an electrically powered blower to circulate air through the enclosed cabin of the vehicle.
It is common for a motor vehicle HVAC system to include ducts carrying inlet air from two separate sources: fresh air from outside the vehicle and recirculated air from within the vehicle. Because of packaging space constraints, it may not be practical to arrange both the recirculation and fresh air ducts so as to feed directly into the fan, so that the fan inlet duct(s) may have a sharp bend close to the fan.
It is a well-recognized problem that abrupt or sharp bends in the inlet duct delivering airflow to a fan may cause flow disruptions. Such flow disruptions may be particularly severe if the sharp bend is close to the blower fan itself. If the flow disruption creates turbulence and/or a low-pressure region at a region of the inlet face of the fan, it may result in undesirable noise under certain airflow and fan speed conditions. Such flow disruptions and related low-pressure regions can also reduce the blower system efficiency.